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Posted: Mon Oct 23, 2017 8:17 pm
by jfender
Hi, last March we had sandstone slabs laid on top of existing concrete. They were not fitted on a new bed of continuous cement but a grid-like structure of cement. It all looked great at the beginning but I think moisture was trapped in the 'holes' where there was no cement. Some of the larger slabs have had 8 dark squares permanently showing, and when the weather is wet the whole patio has a 'checkered' look. It's pretty ugly, and I'm rather disappointed. I'm hoping there might be a sealant that will disguise this look/prevent rain entering these holes, though I guess moisture could also be coming up from underneath? Any advice/thoughts welcome. Thank you. Would love to post a photo but not sure if this is possible on here ??
Posted: Mon Oct 23, 2017 8:39 pm
by seanandruby
Posted: Mon Oct 23, 2017 8:59 pm
by jfender
I wish I had seen this website before getting the work done 
is there not a sealant that would reduce the look of the dark damp patches???
Posted: Tue Oct 24, 2017 9:42 am
by Tony McC
It's reflective staining - no known fix, sadly.
Posted: Tue Oct 24, 2017 10:05 am
by jfender
Thank you for your reply. My patches however are not where the cemet is, but rather above the gaps where there is no cement. Would maybe a colour enhancer sealant be worth trying? A sealant that would prevent moisture from penetrating the slabs and collecting in the gaps? Any advice very welcome.
Posted: Sat Nov 25, 2017 2:16 pm
by Tony McC
No - that's still reflective staining because the mineral carry-through from the mortar spots is limiting the ability of algae/lichens to establish themselves on the flag surface. Think of it as a sort of 'negative' of the more commonly seen reflective staining, so no colour enhancer, no sealant, no blck spot remover, no magic jollop whatsoever is going to rectify the situation.